This research application of the Washington University Mass Spectrometry Resource plans: 1. To continue to provide wide range low resolution GCMS service to local, regional, national, and international biomedical investigators who request this assistance. 2. To continue to provide expert technical assistance, analytical services, and interpretive advice on high precision GCMS selected ion monitoring quantitation (SIM-GCMS) of substrate content and stable isotope enrichment in physiologic samples. 3. To develop a fully automated isotope ratio mass spectrometer (IRMS) to permit rapid, high sample volume operation for measurement of stable isotope enrichment in expired and excreted end products of endogenous substrate metabolism. 4. To develop an isotope ratio monitoring GCMS (IRM-GCMS) for high precision quantitative stable isotope analysis with smaller sample size and less sample preparation than with IRMS and with the ability to measure greater tracer dilution than possible with SIM-GCMS. 5. To establish a high resolution mass spectrometry-fast atom ionization facility for the analysis of high molecular weight or thermally labile molecules. 6. To make the newly developed facilities available for use by qualified investigators at the local, regional, national, and international levels.